1. Field of the Invention
The invention relates to equipment for arc welding, and more specifically, it deals with an apparatus for overhead submerged-arc welding.
2. Description of the Related Art
A large number of operations is performed in the manufacture of welded structures with welding of rotatable annular welds of hollow products with a restricted access to joints being welded from the interior. Such joints include annular joints of closed vessels, annular joints of pipelines, tanks, casings, assembly welds, shell plating seams of ship hulls; longitudinal welds of large-area products which it is difficult to place to a position facilitating welding. Such joints include joints between difficult-to-position webs, segments, three-dimensional and planar sections and other members.
The overhead submerged-arc welding method is characterized by the fact that a consumable electrode and welding bath are turned at 180.degree. in comparison with the downhand submerged-arc welding. Flux and electrode are supplied from bottom up, i.e. as though towards a ceiling. The electrode is supplied through compacted flux.
This is why this welding method will be referred to hereinbelow as overhead submerged-arc welding.
This welding method is referred to as overhead submerged-arc welding also because arc is in the body of metal.
So called overhead welds are produced as a result of such welding.
Overhead welds may be of various types, e.g. penetration overhead welds and sealing overhead welds. There may also be one-pass overhead welds and other overhead welds.
The penetration overhead welds are the welds which are first to be produced in welding a joint and which are located in the top part of sections being welded, on the opposite side of the joint with respect to the electrode supply. Further welding of the joint, i.e. producing further welds, can be carried out by any appropriate known method, the electrode being supplied on the same side as was the case in producing the penetration overhead weld, e.g. in producing inner penetration welds of rotatable annular joints of vessels, tanks, joints between bottom sections of shell plating of ships, and other structures.
The overhead submerged-arc welding of penetration welds mainly allows welding inside vessels in producing rotatable annular welds to be eliminated, and welding can also be avoided in confined spaces in producing straight welds of structures with an access on the side opposite to the ceiling.
The sealing overhead welds are the welds which are first to be produced in welding a joint and which are located in the bottom part of sections being welded on the joint side in the vicinity to the electrode supply. Further welding is carried out by any appropriate known method, the electrode being supplied on the opposite side of the joint as compared with the overhead welding.
In practice, the penetration overhead welds are produced in welding annular and longitudinal joints of structures with a restricted access to joints being welded from the interior.
The sealing welds are produced in welding elongated longitudinal joints of difficult-to-position products such as plate structures made out of segments and other members.
The one-pass overhead welds are the welds produced in welding joints of a limited thickness located over the whole welded sections. No further welding of the joint on either side is required.
Many problems arise in producing sealing and one-pass welds in forming the surface of the finished weld.
The metal in the welding bath formed during arcing by fusion of the metal being welded, electrode material and welding flux is held by the crust of partially melted flux and by forming means. Forming means may be of various configuration and size and may be, e.g. in the form of plates, backings, bars, sliders and other members.
Flux is positively pressed against a welding spot from bottom, and as flux is consumed, its stock is continually replenished. Flux may be supplied for forming the top part of the weld either on the bottom side through the gap between the edges of welded members or from top by any appropriate known method so as to form a filled flux layer. Special forming backings or flux holders may also be used.
Special problems arise in welding joints of large-size cylindrical or like products such as ship hulls and boiler units where especially high quality of welds is required and where the product should be rotated about its axis during welding, and also in welding large-size planar members which it is difficult to place in a position facilitating welding.
Known in the art is an apparatus for overhead submerged-arc welding (SU, A, 1343111), comprising a hopper containing flux pivotally mounted on a pivot pin and accomodating a bowl having its open part facing towards a work being welded. The bowl communicates with a flux supply pipe having inlet and outlet ports and an auger provided in the pipe and having a drive for supplying flux to the bowl and pressing it against the work. A welding nozzle for supplying a consumable electrode extends through the bowl. The apparatus has a pair of copying wheels, one wheel being provided on the front part of the hopper in the welding direction and the other being aligned with a forming means. The hopper supports a welding head. The hopper is mounted on a pivot pin. The same pivot pin supports the other copying wheel and the forming means.
The apparatus is provided with means for turning the hopper about its pivot pin and a means for pressing the second copying wheel and forming means against the work being welded.
The forming means is provided in the vicinity to the nozzle to be located above the brim of the open part of the bowl.
The pivot pin supporting the hopper, forming means and second copying wheel are mounted on an arm provided on a carriage having means for pressing the forming means with the copying member against the work being welded, which comprises a power actuator for moving the carriage towards the work being welded.
The prior art apparatus is so constructed as to allow both absolute value of flux pressure in the bowl and force with which the forming means is pressed against the work being welded and the ratio between them to be varied.
This apparatus allows high-quality overhead submerged-arc welding to be carried out with the desired forming of weld on either side in a broad range of process capabilities and with various types of products being welded.
However, as the forming means and the second copying wheel in this apparatus are mounted on one and the same pivot pin, the forming means is pressed away from the work in case of a substantial convexity of the joint being welded on the under-side of the work. This results in a substantial change in position of the axis of oscillations of the hopper and copying wheel with respect to the surface of the work being welded. This change in position of the axis of oscillations of the hopper results in material fluctuations of preset pressures of flux in the bowl and at different points where the welding bath is formed (upstream of the arc, in the arc zone, and in the welding bath zone and at the point of solidification of the welding bath), i.e. in a disruption of welding as a whole, hence in impaired quality of the welded joint.
In cases of a substantial concavity of the joint being welded on the underside of the work, the forming means is separated therefrom so that a substantial surplus space is formed between the working face of the forming means and the work to disrupt welding.
In addition, in welding products with geometry and assembly errors of joints, the amount of clearance between the bowl and work fluctuates in the zone between the two copying wheels which are in contact with the work so as to result in a change in flux volume available between the bowl and work thus causing fluctuations of flux pressure in the bowl and impaired welding quality as a whole.
Also known in the art is an apparatus for overhead submerged-arc welding (DE, C, 3430394), comprising a suspended pivotally mounted hopper containing flux and accommodating a bowl having its open part facing towards a work being welded and communicating with a flux supply pipe having inlet and outlet ports and an auger provided in the pipe having a drive for supplying flux to the bowl and pressing it against the work. The pipe, auger and drive for supplying flux to the bowl and pressing it against the work from a driven auger feeder. A welding nozzle for supplying a consumable electrode extends through the bowl.
A copying member is provided adjacent to the welding zone and is engageable with the work surface during welding. A forming means provided adjacent to the welding nozzle above the brim of the open part of the bowl is mounted on a suspension for oscillations in its longitudinal and transverse planes. The hopper is pivotally mounted on an arm for rotation about its pivot pin.
The pivot pin supporting the hopper is mounted on the arm which is movable in the direction towards the work being welded.
The same arm supports the forming means and the copying member mounted on pivot pins.
The support pivot pin of the forming means is in the form of a point-like abutment at the end of an arm of a double-arm suspension lever of the forming means. The fulcrum of the double-arm lever is mounted on an arm, the other arm of the lever being connected to a power actuator pivotally attached to the arm. The copying member is in the form of a copying wheel and is mounted on the pivot pin supporting the hopper.
In another embodiment of this apparatus, the pivot pin supporting the forming means comprises a point-like abutment at the end of a mounting arm located adjacent to the welding nozzle.
In this embodiment of the apparatus the copying member is in the form of projections on the face of the forming means; the pivot pin supporting the hopper is located on the side of the forming means remote from the nozzle, and the hopper is provided with a means for moving it with respect to its support pivot pin.
In this apparatus the mounting arm is also mounted on a carriage which is mounted, together with its power actuator, on a driven trolley for moving the whole apparatus in the welding direction.
This apparatus allows permanent contact between the forming means and copying member and the work being welded to be ensured during welding with various assembly errors and deviations from geometry of the joint being welded (e.g., misalignment of plate edges, convexities and concavities, undulations, clearances, and the like).
Therefore, upon any change in position of the forming means during welding caused by an admissible change in profile of the surface of the work being welded at a point of their contact, the eventual action of the forming means upon position of the hopper containing flux is eliminated.
Undesirable oscillations of the hopper upon changes in profile of the surface of the work are thus eliminated, and such welding parameters as thickness of a flux backing and flux pressure in the welding zone are stabilized.
The flux backing is an area of a compacted compressed flux layer which has a preset pressure distributed over the whole area of the flux backing and which is located between the top part of the bowl facing towards the work being welded and the surface of the work being welded to exert a local pressure upon the surface of the joint being welded in the welding zone.
This construction of the apparatus provides conditions for a smooth copying by the forming means of the surface of the joint being welded without jerks and shakes which is necessary for maintaining stable preset values of flux pressure acting upon the welding bath and upon zones in which the welding bath is formed along the joint being welded.
The construction of the prior art apparatus also provides conditions for a smooth copying of the surface of the joint being welded by the forming means and for adjusting position of the hopper with respect to the work being welded which is necessary for carrying out welding of products of different configurations.
In this apparatus, a change in position of the forming means during welding which occurs because of errors in geometry and assembly of the joint being welded does not cause a change in position of the pivot pin supporting the hopper with respect to the surface of the work being welded.
In welding with such an apparatus, owing to the creation and maintenance at a constant level of preset flux pressures at various points along the joint being welded (upstream of the arc, in the zone of the arc and welding bath, and in the zone downstream of the welding bath and up to the formed weld), the possibility of automatic conduct of welding of overhead welds and production of high-quality welded joints is ensured.
This apparatus makes it possible to carry out welding with a desired formation of weld reinforcement on either side of the joint over a broad range of process capabilities with a wide range of welded products and with large errors of assembly of joints before welding (misalignment of plate edges, undulations, convexities, and concavities, taper of bottoms in welding annular joints, e.g. in boilers and railway tanks, and the like welded structures).
Investigations showed that the flux backing thickness (the space between the surface of the outlet port of the bowl and the surface of the joint being welded filled with flux) should be kept constant during automatic overhead submerged-arc welding during the entire welding period. Theis is required to provide conditions during welding for the maintenance of constant preset flux pressures at various points along the joint being welded (upstream of the arc, in the zone of arc and welding bath, and in the zone of solidification of the weld) and for retaining the welding bath at the level of the joint being welded.
In the abovedescribed apparatus, flux moves during welding along a close-loop circuit: supply of a preset amount of flux by the auger feeder from the hopper to the bowl and spillage of (non-melted) flux that did not participate in welding from the bowl with subsequent supply thereof during the next cycle.
Flux is comminuted upon every such cycle (grading composition of flux changes) which results, as shown by investigations, in a decrease in throughput capacity of the auger feeder (decrease in the amount of flux supplied by the auger to the bowl per unit of time), hence in a decrease in the flux backing thickness if drives of the flux supply means (auger rotation) and of the driven trolley in welding longitudinal joints or rotary drive for rotating the product make use of constant speed electric motors (e.g. induction motors which are mainly used under industrial heavy-duty conditions).
However, in welding elongated welds at a constant speed, i.e. in applications where multiple passage and comminution of flux in the abovedescribed close-loop circuit occurs in the auger feeder, the prior art apparatus does not make it possible to keep the flux backing thickness constant. Owing to comminution of flux and reduction of its particle size, the flux backing thickness decreases as length of elongated welds and the number of close-loop movement cycles of flux increase. The reduction of the flux backing thickness results in fluctuations of preset flux pressures during welding at various points along the joint being welded and in a disruption of welding as a whole.
Welding should be interrupted in such cases, and the apparatus should be stopped. The comminuted (pulverulent) flux should be replaced with fresh flux of optimum grading composition. The end crater of the weld is then repaired after replacement of flux, and only then further welding of the elongated joint can continue. Quality of welding of such joints is rather low because of interruptions, presence of repaired zones, and other defects.
In addition, this apparatus does not allow preset thickness of the flux backing to be set up or be varied if necessary in welding elongated joints at a constant welding speed (e.g. in welding structures with different plate thicknesses along one and the same joint such as in making railway tanks and other products) and flux cannot be replaced with a flux of other grade or type (fine-grained, ceramic, and the like).
In all such cases process of overhead submerged-arc welding becomes complicated.
Therefore, to enhance and facilitate process of overhead submerged-arc welding at a constant welding speed and to produce a constant and high quality of welds along the entire length of a joint being welded, it is necessary, as flux is comminuted after each close-loop flux movement cycle, to increase flux supply by the auger feeder to the welding zone (bowl) in such a manner that the amount of flux supplied to the welding zone during welding at a constant welding speed remain unchanged.
Supplying a constant amount of flux to the bowl ensures the maintenance of a constant preset flux backing thickness in welding elongated welds at a constant welding speed which is necessary for high-quality overhead submerged-arc welding.